1. Cut away the front of the chest, exposing the heart and lungs. Find on each side of the heart a nerve which pa.s.ses by the side of the pericardium to the diaphragm. These nerves a.s.sist in controlling respiration and are called the _phrenic_ nerves. Find other nerves going to different parts of the thorax.

2. Remove the heart and lungs. Find in the back part of the thoracic cavity, on each side of the spinal column, a number of small "knots" of nervous matter joined together by a single nerve. These are sympathetic ganglia. Where the neck joins the thorax, find two sympathetic ganglia much larger than the others.

3. Cut away the skin from the shoulder and upper side of the fore leg. By separating the muscles and connective tissue where the leg joins the thorax, find several nerves of considerable size. These connect with each other, forming a network called the _brachial plexus_. From here nerves pa.s.s to the thorax and to the fore leg.

4. From the brachial plexus trace out the nerves which pa.s.s to different parts of the fore leg. In doing this separate the muscles with the fingers and use the knife only where it is necessary to expose the nerves. Note that some of the branches pa.s.s into the muscles, while others connect with the skin.

5. Remove the skin from the upper portion of one of the hind legs and separate the muscles carefully until a large nerve is found. This is one of the divisions of the _sciatic_ nerve. Carefully trace it to the spinal cord, cutting away the bone where necessary, and find the connections of its branches with the cord. Then trace it toward the foot, discovering its branches to different muscles and to the skin.

6. Unjoint the neck and remove the head. Examine the spinal cord where exposed. Cut away the bone sufficiently to show the connection between the cord and one of the spinal nerves. On the dorsal root of one of the nerves find a small ganglion. What is it called?

7. Fasten the head to a small board and remove the scalp. Saw through the skull bones in several directions. Pry off the small pieces of bones, exposing the upper surface of the brain. Study its membranes, convolutions, and divisions.

8. With a pair of bone forceps, or nippers, break away the skull until the entire brain can be removed from the cavity. Examine the different divisions, noting the relative position and size of the parts.

9. With a sharp knife cut sections through the different parts, showing the positions of the "gray matter" and of the "white matter."

NOTE.-If the entire cla.s.s is to examine one specimen, it is generally better to have the dissecting done beforehand and the parts separated and tacked to small boards. This will permit of individual examination.

Sketches of the sciatic nerve, brachial plexus, and of sections through the brain and spinal cord should be made.

*Location of Nerves in the Body.*-Several of the nerves of the body lie sufficiently near the surface to be located by pressure and are easily recognized as sensitive cords. Slight pressure from the fingers reveals the presence of nerves in the grooves of the elbow (the crazy bone), between the muscles on the inner side of the arm near the shoulder, and in the hollow part of the leg back of the knee. These are all large nerves.

Small nerves may be located in the same manner in the face and neck.

CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM

In the preceding chapter was pointed out the method by which the different parts of the body are brought into communication by the neurons or nerve cells. We are now to study the means whereby the neurons are made to control and coordinate the different parts of the body and bring about the necessary adjustment of the body to its surroundings. This work of the neurons naturally has some relation to their properties.

*Properties of Neurons.*-The work of the neurons seems to depend mainly upon two properties-the property of irritability and the property of conductivity. _Irritability_ was explained, in the study of the muscles (page 243), as the ability to respond to a stimulus. It has the same meaning here. The neurons, however, respond more readily to stimuli than do the muscles and are therefore more irritable. Moreover, they are stimulated by all the forces that induce muscular contraction and by many others besides. They are by far the most irritable portions of the body.

_Conductivity_ is the property which enables the effect of a stimulus to be transferred from one part of a neuron to another. On account of this property, an excitation, or disturbance, in any part of a neuron is conducted or carried to all the other parts. Thus a disturbance at the distant ends of the dendrites causes a movement toward the cell-body and, reaching the cell-body, the disturbance is pa.s.sed through it into the axon. This movement through the neuron is called the _nervous impulse_.

*Purpose of the Impulse. *-Though the nature of the nervous impulse is not understood, (103) its purpose is quite apparent. It is the means employed by the nervous system for controlling and coordinating the different parts of the body. The arrangement of the neurons enables impulses to be started in certain parts of the nervous system, and the property of conductivity causes them to be pa.s.sed _as stimuli _to other parts. This enables excitation at one place to bring about action at another place.

Acting as stimuli, the impulses seem able to produce two distinct effects: first, to throw resting organs into action and to increase the activity of organs already at work; and second, to diminish the rate, or check entirely, the activity of organs. Impulses producing the first effect are called _excitant_ impulses; those producing the second effect, _inhibitory_ impulses.

*Functions of the Parts of Neurons.*-The _cell-body_ serves as a nutritive center from which the other parts derive nourishment. Proof of this is found in the fact that when any part of the neuron is separated from the cell-body, it dies, while the cell-body and the parts attached to the cell-body may continue to live. In addition to this the cell-body probably reenforces the nervous impulse.

The _dendrites_ serve two purposes: first, they extend the surface of the cell-body, thereby enabling it to absorb a greater amount of nourishment from the surrounding lymph; second, they act as _receivers of stimuli_ from other neurons. The same impulse does not pa.s.s from one neuron to another. An impulse in one neuron, however, is able to excite the neuron with which it makes an end-to-end connection, so that a series of impulses is produced along a given nerve path (Fig. 129).

The special _function of the axon_ is to transmit the impulse. By its length, structure, and property of conductivity it is especially adapted to this purpose. The axis cylinder, however, is the only part of the axon concerned in the transmission. The primitive sheath and the medullary layer protect the axis cylinder, and, according to some authorities, serve to insulate it. The medullary sheath may also aid in the nourishment of the axis cylinder.

*Nerve Stimuli.*-While the properties of irritability and conductivity supply a necessary cause for the production and transmission of nervous impulses, these alone are not sufficient to account for their origin. An additional cause is necessary-a force not found in the nerve protoplasm, but one which, by its action on the protoplasm, makes it produce the impulse. In this respect, the neuron does not differ essentially from the cell of a muscle. Just as the muscle cell requires a stimulus to make it contract, so does the neuron require a stimulus to start the impulse.

Hence, in accounting for the activities of the body, it is not sufficient to say they are caused by nervous impulses. We must also investigate the _nerve stimuli_-the means through which the nervous impulses are started.

Most of these are found outside of the body and are known as external stimuli.

*Action of External Stimuli.*-In the arrangement of the nervous system the most favorable conditions are provided for the reception of external stimuli. Not only do vast numbers of neurons terminate at the surface of the body,(104) but they connect there with delicate structures, called _sense organs_. The purpose of the sense organs is to _sensitize_ (make sensitive) the terminations of the neurons. This they do by supplying special structures through which the stimuli can act to the best advantage upon the nerve endings. Moreover, there are different kinds of sense organs, and these cause the neurons to be sensitive to different kinds of stimuli. Acting through the sense organs adapted for receiving them, light, sound, heat, cold, and odors all act as stimuli for starting impulses. Indeed, the arrangement is so complete that the nervous system is subjected to the action of external stimuli in some form practically all the time. The work of the sense organs is further considered in Chapters XX, XXI, and XXII.

*How External Stimuli act on Internal Organs.*-For stimulating the neurons not connected with the body surface we are dependent, so far as known, upon the nervous impulses. An impulse started by the external stimulus goes only so far as its neuron extends. But it serves as a stimulus for the neuron with which the first connects and starts an impulse in this connecting neuron, the point of stimulation being where the fiber terminations of the first neuron make connection with the dendrites of the second. This impulse in turn stimulates the next neuron, and so on, producing a series of impulses along a given nerve path. In this way the effect of an external stimulus may reach and bring about action in any part of the body. This is in brief the general plan of inducing action in the various organs of the body. This plan, however, is varied according to circ.u.mstances, and at least three well-defined forms of action are easily made out. These are known as _reflex action, voluntary action_, and _secondary reflex action_.

*Reflex Action.*-When some sudden or strong stimulus acts upon the nerve terminations at the surface of the body, an immediate response is frequently observed in some quick movement. The jerking away of the hand on accidentally touching a hot stove, the winking of the eyes on sudden exposure to danger, and the quick movements from slight electrical shocks are familiar examples. The explanation of reflex action is that external stimuli start impulses in neurons terminating at the surface of the body and these, in turn, excite impulses in neurons which pa.s.s from the spinal cord or brain to the muscles (Fig. 138). Since there is an apparent turning back of the impulses by the cord or brain, the resulting movements are termed _reflex_.(105)

[Fig. 138]

Fig. 138-*Diagram ill.u.s.trating reflex action of an external organ.*

*Reflex Action and the Mind.*-If one carefully studies the reflex actions of his own body, he will find that they occur at the time, or even a little before the time, that he realizes what has happened. If a feather is brought in contact with the more sensitive parts of the face of a sleeping person, there is a twitching of the skin and sometimes a movement of the hand to remove the offending substance. Surgeons operating upon patients completely under the influence of chloroform, and therefore completely unconscious, have observed strong reflex actions. These and other similar cases indicate clearly that reflex action occurs _independently_ of the mind-that the mind neither causes nor controls it.

If a further proof of this fact were needed, it is supplied by experiments upon certain of the lower animals,(106) which live for a while after the removal of the brain. These experiments show that the nervous impulses that produce reflex action need only pa.s.s through the spinal cord and do not reach the cerebrum, the organ of the mind.

*The Reflex Action Pathway.*-By study of the impulses that produce any reflex action, a rather definite pathway may be made out, having the following divisions:

1. _From the surface of the body to the central nervous system_ (usually the spinal cord). This, the _afferent_ division, is made up of di-axonic neurons, and these have (in the case of the spinal nerves) their cell-bodies in the dorsal root ganglia (page 295). They are acted upon by external stimuli, while their impulses in turn act on the neurons in the spinal cord.

2. _Through the central system_ (spinal cord or base of brain). This, the _intermediate_ division, may be composed of mon-axonic neurons, or it may consist of branches from the afferent neurons. In the case of separate neurons, these are acted upon by impulses from the afferent neurons, while their impulses serve in turn as stimuli to other neurons within the cord (Fig. 129).

3. _From the central nervous system to the muscles._ This, the _efferent_ division, is made up of mon-axonic neurons. Most of these have their cell-bodies in the gray matter of the cord, while their fibers pa.s.s into the spinal nerves by the ventral roots.(107) They may be stimulated by impulses either from the intermediate neurons, or from branches of the afferent neurons. Their impulses reach and stimulate the muscles.

*Reflex Action in Digestion.*-The flowing of the saliva, when food is present in the mouth, is an example of reflex action. In this case, however, the organ excited to activity is a gland instead of a muscle. The food starts the impulses, and these, acting through the bulb, reach and stimulate the salivary glands. In a similar manner food excites the glands that empty their fluids into the stomach and intestines, and stimulates the muscular coats of these organs to do their part in the digestive process. To a considerable extent, neurons having their cell-bodies in the sympathetic ganglia are concerned in these actions (Fig. 139).

[Fig. 139]

Fig. 139-Diagram ill.u.s.trating reflex action in its relation to the food ca.n.a.l. The nerve path in this case includes sympathetic neurons.

*Reflex Action in the Circulation of the Blood.*-On sudden exposure to cold, the small arteries going to the skin quickly diminish in size, check the flow of blood to the surface, and prevent too great a loss of heat. In this case, impulses starting at the surface of the body are transmitted to the bulb and then through the efferent neurons to the muscles in the walls of the arteries. In a somewhat similar manner, heat leads to a relaxation of the arterial walls and an increase in the blood supply to the skin.

Other changes in the blood supply to different parts of the body are also of the nature of reflex actions. As in the work of digestion, neurons having their cell-bodies in the sympathetic ganglia aid in the control of the circulation.

*Purposes of Reflex Action.*-The examples of reflex action so far considered ill.u.s.trate its two main purposes-(1) protection, and (2) a means of controlling important processes.

The pupil has but to study carefully the reflex actions of his own body for a period, say of two or three weeks, in order to be convinced of their protective value. He will observe that portions of his body have, on exposure to danger, been moved to places of safety, while in some instances, like falling, his entire body has been adjusted to new conditions. He will also find that reflex action is quicker, and for that reason offers in some cases better protection, than movements directed by the mind. In digestion and circulation are found the best examples of the control of important processes through reflex action.

*Voluntary Action.*-It is observed that reflex action, in the sense that it has so far been considered, is not the usual mode of action of the external organs, but is, instead, a kind of emergency action, due to unusual conditions and excitation by strong stimuli. Voluntary actions, on the other hand, represent the ordinary, or normal, action of these organs.

They comprise the movements of the body of which we are conscious and which are _controlled by the mind_. But while they are of a higher order than reflex actions and are under _intelligent_ direction, they are brought about in much the same manner.

*Voluntary Action Pathways* differ in but one essential respect from those of reflex action. They pa.s.s through the cerebrum, the organ of the mind (Fig. 140). This is necessary in order that the mind may control the action. From all portions of the body surface, afferent pathways may be traced to the cerebrum; and from the cerebrum efferent pathways extend to all the voluntary organs. A complex system of intermediate neurons, found mostly in the brain, join the afferent with the efferent pathways. The voluntary pathways are not distinct from, but include, reflex pathways, a fact which explains why the same external stimulus may excite both reflex and voluntary action (Fig. 141).

[Fig. 140]

Fig. 140-*Diagram of a voluntary action pathway.*